This invention relates to electrophotographic development utilizing magnetic toner particles which are applied from magnetic brush to the electrostatic latent image without use of a carrier material.
A vast majority of the electrographic copying processes in use today involve creation on a suitable recording medium of an electrostatic charge pattern corresponding to a pattern of light and shadow to be reproduced and the development of that pattern by deposition of marking material on the recording medium according to forces generated by such electrical potential pattern. Xerography is the most widely known of these techniques. The substrate may be photoconductive, such as in the case of selenium as taught by Carlson in U.S. Pat. No. 2,297,691, or may be a conventional insulating substrate overlying a photoconductor layer, as described in Watanabe, U.S. Pat. No. 3,536,483, to name a few examples.
After creation the electrical potential pattern is generally developed by means of a finely divided developer powder thus giving form to the hitherto latent electrostatic image. In a common technique a fine, insulating, electroscopic powder is cascaded over the electrical potential pattern bearing member. The powder is, in the conventional use, triboelectrically charged to a definite polarity and deposits preferentially in regions of the surface where there is a preponderance of charge of the opposite polarity. The triboelectric charge is caused by presence of carrier beads in the powder mix. This technique of development is called cascade development.
In another form of development, called magnetic brush development, magnetic carriers or magnetic toners are employed. In this technique a magnetic force is used to provide adherence of the toner-carrier mixture to a support member which is then presented to the image bearing member. In comparison to cascade development, magnetic brush development fills in solid areas better, is more compact, and does not depend on gravity to present the toner to the surface, a factor which allows freedom in locating the developer station.
In yet another form of electrostatic charge pattern development, a conductive one-component toner is used by bringing a conductive support member bearing a layer of fine conductive toner powder into contact with the charge pattern bearing member as in U.S. Pat. No. 3,166,432 to Gundlach. In this case the toner is held to the support member by van der Waal's forces and the conductive support member is held at a bias potential during development. This technique fills in solid areas and requires only one component in the developer material.
A further method of developing an electrostatic charge pattern is to employ an electroscopic toner suspended in a liquid. With the proper choice of materials, the toner becomes charged to a definite polarity when dispersed in the liquid. When the electrostatic charge pattern bearing member is brought into contact with the liquid suspension, the toners deposit where there is a preponderance of charge of the opposite polarity as in cascade development.
While all of the above techniques have certain advantages in particular situations, each one suffers from disadvantages which impair their utility in actual machines.
In the conventional cascade development technique the toner-carrier combination has a definite charge polarity and is not reversible without charging the toner or the carrier. Thus, positive and negative developed images cannot easily be made. Also the images are hollow and solid areas are not filled in resulting in low-fidelity development compared to the original charge pattern. The triboelectric properties of the toner, while necessary to development, cause severe problems. Uneven charging of the toners causes backgrounding as do the uneven forces between carrier and toner result in varying threshold levels from toner to toner. Also, since the toner does not retain its charge for long periods of time, during cascading some toners escape the development region and enter other parts of the apparatus causing mechanical problems. These problems, coupled with the inherent problem of using a two-component system where only one component is depleted, definitely limit the utility of such techniques.
The magnetic brush development, as it also uses carrier, suffers from some of the above mentioned disadvantages although it overcomes others. As mentioned above, this technique is less efficient but helps to fill in solid areas. However, it still requires triboelectric toners, which have the concomitant problems mentioned above. Also, due to the mechanical brusning action and other electrical characteristics, this technique usually results in high background desposition and poor machine latitude.
A system utilizing two different toners has been proposed in U.S. Pat. No. 3,262,806 wherein the toners acquire opposite charges when mixed with a carrier. This development system may be used for simultaneous development of images and background in two colors. This system is difficult to control as the two toners are not used at the same rate and it becomes unbalanced.
The process described in Gundlach, U.S. Pat. No. 3,166,432, has many advantages over the above mentioned cascade type techniques. However, it suffers from drawbacks which limit its applicability. The van der Waal's forces, which act to adhere the toner onto the conductive support member, are a counterforce to the image producing electric force generated by the electrostatic charge pattern, and as such must be selectively overcome to have toner deposited. The van der Waal's forces are weak and non-uniform from one toner to the next. Also high contrast is difficult to achieve. The fact that the van der Waal's forces are not under direct control but subject largely to the surface properties of the materials involved makes the system highly susceptible to alteration of development properties upon wearing of the involved surfaces or variations in ambient conditions of temperature and humidity.
In a liquid development technique most of the problems of cascade development are present in addition to others unique to a liquid system. Also, as in the case of cascade development, the charge on a given toner is not well controlled, resulting in high background deposition, poor machine latitude, and a characteristic splotchiness in large dark or grey areas. The inherent problems of the handling liquids, usually solvents, in a machine are also present.
There have been suggested systems for magnetic development not utilizing the carrier material. One such system was described in U.S. Pat. No. 2,846,333 to Wilson. Wilson et al disclosed the use of magnetic brush to apply toner particles formed of ferrites and resin material to develop electrostatic latent images. The difficulty with this process was the conductivity of the toner makes electrostatic transfer difficult.
A further development of electrostatic development without carriers is illustrated by Kotz, U.S. Pat. No. 3,909,258 wherein an electrostatic development process utilizing a magnetic brush without carrier is illustrated. A toner suitable for use in the Kotz process is disclosed in U.S. Pat. No. 3,639,245 to Nelson wherein a dry toner powder having specific electric conductivity is disclosed. The toner of Nelson is formed by blending magnetite with the resin and then after blending pulverizing to a small particle size. The particles are then mixed with conductive carbon black which is embedded in the surface of the particle to make it conductive and then a small particle size SiO.sub.2 agent is mixed into the toner to improve the flowability. The toner of Nelson suffers the disadvantage that it does not transfer efficiently from a photoconductive substrate to plain bond paper.
Therefore there is a continuing need for toners suitable for use in carrierless development systems both with and without the use of magnetic development. There is a need for toners suitable for high speed development that also have good electrostatic transfer characteristics for high transfer efficiency from a photoconductive surface to plain bond paper.